The Sun constantly releases a stream of charged particles known as the solar wind, which affects space weather and can influence planetary atmospheres. Measuring how fast the solar wind moves near the Sun is challenging because spacecraft cannot safely enter those regions.

In this study, we developed a way to estimate solar wind speed by analyzing how spacecraft radio signals change as they pass through the Sun’s atmosphere. We first applied this method to S-band signals from India’s Mars Orbiter Mission (MOM) in 2021, when the spacecraft was behind the Sun from Earth’s perspective. We found that the spread in radio signal frequency called Doppler spectral broadening can be linked to the speed of the solar wind.

We then adapted the method to X-band data from Japan’s Akatsuki spacecraft, using observations from 2016 and 2022. These allowed us to probe a wider range of distances from the Sun (1.4 to 10 solar radii) and different latitudes, including the poles. We observed faster solar wind flows in polar regions.

This research offers a new, remote-sensing approach to study the solar wind, complementing direct measurements from missions near the Sun.